I would make sure you find out WHY he is lossing so much weight then I would try to get it back on. You don't want to increase his food to much because you don't want him to gain to fast. Slow weight gaining is alway better. If everything is better health wise then I would increase the food intake a little at a time. Feed him whatever the amount is then in 2 days increase it by maybe 1/2 cup of dry food and see how he is doing. if he eats all that and you don't see a gain then increase a little again. This will take time. You won't see results right away.